For developing a lighting device into various lighting uses, the technique of emitting light of three primary colors of red, green and blue is indispensable. In this regard, a light emitting diode (LED) has not been developed to various uses because blue among the three primary colors is deficient due to delay in completion of blue LED.
However, after a nitride-based blue LED was invented in 1990's, products using an LED have been developed to various lighting uses including a backlight of a liquid crystal monitor, a backlight of liquid crystal television, and household use as well as a signal.
In recent years, a liquid crystal television equipped with an LED backlight rapidly come into wide use owing to price drop. Further, since lighting using an LED has a merit of better friendliness to environment over conventional lighting, owing to lower power consumption, space saving and free from mercury, it has rapidly become widespread since the summer of 2009 when lighting using an LED appeared on the market at a considerably lower price than before.
By the way, for lighting and a backlight of a liquid crystal television, white light is used. White light is generally realized by combination of a blue LED and a YAG (yttrium aluminum garnet) yellow phosphor, or by combination of a blue LED and a green phosphor and a red phosphor. In other words, a blue LED is required in any combination for realizing white light. For this reason, a method of massively manufacturing a blue LED of high brightness at a low cost is demanded.
In general, in a light-emitting layer of an LED or a laser diode (LD) of short wavelength such as blue, blue green or the like, gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and mixed crystals thereof are used, and further, a group III-V compound semiconductor containing nitride as a group V element is used. One example of a conventional blue LED will be described below.
FIG. 10 is a schematic sectional view showing one example of a conventional double hetero junction type blue LED. As shown in FIG. 10, the conventional double hetero junction type blue light-emitting element is formed by stacking, on a substrate 101 formed of sapphire, a lower clad layer 102 formed from a layer of n-type GaN layer doped with Si; a light-emitting layer 103 formed from an InGaN layer; an upper clad layer 104 formed of a p-type AlGaN doped with Mg; and a contact layer 105 in this order.
On contact layer 105, an electrically conductive film 108 is formed, and on a part of electrically conductive film 108, a p-type electrode 106 is provided. On the other hand, on a part on lower clad layer 102, an n-type electrode 107 is provided. When a current is injected from p-type electrode 106, the current is diffused in the planar direction of electrically conductive film 108. Then, the current injected from p-type electrode 106 is injected over a large area in upper clad layer 104 and light-emitting layer 103 to cause emission of light in light-emitting layer 103.
The light emitted upward in light-emitting layer 103 penetrates upper clad layer 104, contact layer 105, and electrically conductive film 108 and is taken outside. By using a highly light transmissive material such as ITO as electrically conductive film 108, it is possible to reduce the light loss when the light emitted in light-emitting layer 103 passes through electrically conductive film 108. Further, since electrically conductive film 108 formed of ITO has lower resistance than contact layer 105, diffusion of the operation current to the wide range for obtaining light emission is promoted, and the light-emitting area is extended, so that light-emitting efficiency can be improved.
On the other hand, electrically conductive film 108 formed of ITO exhibits a relatively high sheet resistance of greater than or equal to 20Ω/□ and less than or equal to 60Ω/□, and the sheet resistance varies depending on the site in the same. Therefore, such a problem arises that driving voltage Vf of the compound semiconductor light-emitting element becomes high, or light emission in the light-emitting layer is not uniform.
For collectively solving these problems, it is ideal to make the sheet resistance of electrically conductive film 108 less than or equal to 20Ω/□, preferably less than or equal to 10Ω/□. For reducing the sheet resistance of electrically conductive film 108, a measure of increasing the carrier density of electrically conductive film 108 by making a crystallized oxygen defective condition in electrically conductive film 108 is conceivable.
However, as the carrier density increases, the work function of electrically conductive film 108 decreases, and the potential on the side of the electrically conductive film at the interface between electrically conductive film 108 and the contact layer increases. Accordingly, a hole is difficult to be injected into the contact layer from the electrically conductive film, and as a result, the problem arises that the contact resistance between the electrically conductive film and the contact layer is high.
Also a measure of reducing the sheet resistance by conducting annealing on the electrically conductive film to increase the crystallinity of the electrically conductive film is conceivable. However, by the annealing, the binding condition at the interface between the electrically conductive film and the contact layer changes, and a stable binding condition of Ga—O, N—O, a compound of H or the like is impaired, and also in this case, the contact resistance is high.
In light of this, in Japanese Patent Laying-Open No. 2007-287786 (PTL 1), an attempt is made to reduce the sheet resistance of the electrically conductive film while keeping the contact resistance low by conducting two-step annealing including first annealing and second annealing. In the first annealing in PTL 1, the contact resistance of electrically conductive film 108 is decreased by conducting the annealing in an atmosphere containing oxygen at a temperature of greater than or equal to 250° C. and less than or equal to 600° C. In the subsequent second annealing, the sheet resistance of electrically conductive film 108 is reduced by conducting the annealing in an atmosphere not containing oxygen at a temperature of greater than or equal to 200° C. and less than or equal to 500° C.    PTL 1: Japanese Patent Laying-Open No. 2007-287786